The present inventors have found that vinyl compounds excellent in curability, dielectric characteristics and heat resistance are obtained by introducing vinyl groups into terminals of bifunctional phenylene ether oligomers which inherit excellent dielectric characteristics and heat resistance from polyphenylene ether resins (see JP-A-2004-59644 and JP-A-2004-67727). The present inventors have also found industrially-advantageous processes of producing the above vinyl compounds in which the content of ionic impurities is extremely small (see JP-A-2005-126523 and JP-A-2006-28111). However, in these processes, it takes a long period of time to remove a salt generated in the step of synthesis so that there is a problem about poor producibility.